First you will need to desolder the + and - battery terminals

As there will still be some power in the batteries, be careful not to short any part of the circuit while desoldering

After removing all the solder, flip over the housing and gently pry the batteries upwards. There is some glue holding them down so you do have to put some effort in.

Be very careful not to puncture the battery. Use a plastic spudger rather than a metal one if you can